Rechargeable batteries, also known as secondary batteries, have in recent years become increasingly important as vehicle-mounted power sources and as power sources for personal computers and mobile devices. In particular, lithium secondary batteries (typically a lithium ion secondary battery), which are lightweight and provide a high energy density, are expected to be preferentially used as large, vehicle-mounted power sources. In this type of lithium secondary battery, charge and discharge proceed through exchange of lithium (Li) ions between the positive electrode and negative electrode that are constituent components of the battery, and the positive electrode material (the positive electrode active material) is the most important component for determining battery performance. For example, the discharge capacity is determined by how much Li ion can be inserted into and extracted from the positive electrode active material.
Due to this, various investigations have been carried out in pursuit of positive electrode active materials that can be expected to realize higher discharge capacities (for example, Patent Literature 1). Titanium composite oxides, e.g., Li2NiTiO4 and Li2MnTiO4, are one example of such positive electrode active materials. For example, the titanium composite oxide represented by Li2NiTiO4 has a theoretical capacity of around 150 mAh/g for the one-electron reaction, but since a two-electron reaction is possible here, it can be expected to achieve a capacity as high as about 300 mAh/g in such a case.